1. Field of the Invention
The present invention relates in general to a method and apparatus for reading and programming an ECL PROM and in particular to a method and apparatus which uses the same ECL address buffers and the decoders coupled thereto for reading and programming the ECL PROM.
2. Description of Prior Art
An ECL PROM is fabricated with a plurality of rows and columns of fuses. Each of the fuses is located in the PROM at a position which is uniquely identified by a row address and a column address.
To program an ECL PROM. a particular fuse is selected by applying a corresponding row and column address to row and column address buffers which are coupled to row and column address decoders, respectively. After the address of the fuse has been decoded and the fuse has been selected, a current which has a magnitude sufficient to blow the fuse is caused to flow through the fuse, blowing the fuse.
To read the data stored in an ECL PROM, the location of one or more of the fuses is addressed also by applying a row and column address of the fuse to a row and column address buffer and decoder circuit coupled thereto. If an addressed fuse had not been blown during programming of the ECL PROM, a first predetermined output is generated from the PROM. If, on the other hand, the addressed fuse had been blown during programming of the ECL PROM, a second predetermined output is generated. The first and second predetermined outputs are complementary and represent either a logical high or a logical low.
Heretofore, the address buffer and decoder circuits used in the operating mode of a conventional ECL PROM have comprised ECL circuits and the address buffer and decoder circuits used in the programming mode of the conventional ECL PROM have comprised TTL circuits.
The above-described use of different buffer and decoder circuits for operating and programming an ECL PROM has certain disadvantages. First, it requires a significant amount of space to accommodate two complete addressing circuits. Second, the algorithms required in both the reading and programming modes are undesirably complicated due to the need for different input levels in the two modes.